1. Field of the Invention
This invention generally relates to information erasure by magnetic degaussing fields of effective strength and multi-dimensional direction applied to magnetic tapes and disks traversing a pathway. In particular, this invention specifically relates to apparatus with multiple and generally duplicate magnetic field generating elements of simple geometry, capable of generating and concentrating multiple interacting flux density fields, predisposed such that media passage causes a range of erase conditions over all points within that media, thereby addressing geometric and material variations internal to the media.
2. Description of the Prior Art
More common in the prior art are magnet means placing two or more poles proximal to one side or both sides of the media. Electromagnet cores in the form of extruded xe2x80x9cUxe2x80x9d or xe2x80x9cExe2x80x9d profiles promote efficient application of windings, such that poles form at the ends of the core appendages for projection of magnetic flux into the media. Problems arise with magnet poles applied only to one side of thick media, as magnetic strength or flux density tends toward inverse proportion with the square of distance from a source such as a pole.
Many forms of prior art bulk degaussers predispose magnetic poles on both sides of magnetic media. Most prior art operates with sets of like poles facing each other across the gap provided for bulk degaussing magnetic media in order to obtain magnetic flux parallel to a plane of mirror image symmetry. U.S. Pat. No. 4,730,230 discloses switching means to change electromagnet operation from like poles to unlike poles facing, yielding flux directed parallel to and perpendicular to the symmetry plane at different times. U.S. Pat. No. 3,023,280 specifies connections that reverse current in one of four coils predisposed about legs of otherwise symmetrically disposed xe2x80x9cExe2x80x9d cores, leading to a known commercial practice exhibiting small magnet size in relation to capability. U.S. Pat. No. 5,416,664 discloses offsetting two electromagnetic xe2x80x9cUxe2x80x9d cores from alignment of unlike poles closest to each other. All of these examples demonstrate the advantages of less symmetric field generating apparatus as compared to more easily constructed and commonly applied apparatus having multiple mirror-image symmetry.
In the interest of protecting information recorded on magnetic media, means placing the media in close proximity to even a single strong magnet can impede recovery of information by ordinary means. Depending on the direction of magnetization of the magnetic material, U.S. Pat. No. 5,666,413 may disclose a single pole, permanent magnet degausser. Depending of the material used for the frame of that invention, it may provide a path of low magnetic reluctance for purposes of returning flux from the working pole to an unlike pole of the magnet distal to the pathway for magnetic disks defined by the frame.
Common forms for information storage on magnetic media include disks and tape wound on reels. In themselves, circular tracks on disks and anisotropic magnetic coatings on nearly circular layers of tape wound on reels exhibit a range of directions. The prior art cited thus far either cannot apply more than fringing flux everywhere in the circular direction of tracks, or else requires more than simple linear motion of the media with respect to the magnetic field. That motion might be accomplished by a human with some skill and diligence, as in the case of a hand-held degausser, or with an automated sequence of motions generally involving rotation either of the media or of magnets generating the degaussing fields.
Problems arise in attempting to configure a complex-motion degausser to a wide variety of media. For example, media can have multiple axes of circularity, as in the case of a partially rewound, 2-reel tape cassette. Some disk drive housings contain the axis of circular disks but do not reveal its location. Also, application of magnetic flux only in the direction of tracks does not ensure erasure of locally magnetized regions having off-track components, as result both from perpendicular components of magnet record head fields, and from fringes along edges of record heads. All but the simplest prior art cited thus far attempts exposure to a variety of magnetic flux directions.
Phased excitation of large, orthogonal coils specified in U.S. Pat. No. 4,617,603 electrically rotates the direction of a magnetic field equal to the media size. Since the electrically rotating field exists everywhere in the media at once, no media motion is required. But phased, media-sized fields imply control over a great quantity of energy, as described in U.S. Pat. No. 5,969,933. Adding a third orthogonal winding to the cavity degausser enables field generation in any direction, but again at added cost and complexity.
U.S. Pat. No. 4,423,460 also discloses means to degauss by electrical rotation of media-sized fields, in this case by phased excitation of multi-pole cores somewhat larger than the target media.
U.S. Pat. No. 4,157,581 discloses rotating, multiple-pole permanent magnets predisposed with mirror-image symmetry so that like poles face each other across a gap admitting a tape cassette intended to be translated through the field that they generate. Given enough rotation per unit translation, all points of tape on either reel will see field in the tangential direction of tracks, but symmetry imposes zero strength in the transverse direction across the tape along a center plane.
U.S. Pat. No. 5,132,860 further discloses rotation of permanent magnets disposed about a degaussing gap at different speeds, leading to a conditions alternating from like poles facing and flux directed parallel to the symmetry plane, to unlike poles facing and flux directed perpendicular to that plane.
Fixed or rotating magnets arrayed for degaussing with simple, linear media motion can become bulky. The Electro-matic Products Co. model HE15VB exemplify the extremes of dimensions, weight and input power applied in the prior art of bulk degaussing large sizes of high coercivity media on a linear conveyor.
A particular challenge in the adaptation of a general-purpose bulk degausser to the erasure of entire hard disk drives arises because certain massive ferromagnetic components of such disk drives experience strong force interactions with the degaussing magnets. Means to turn the magnets off to relieve such forces as practiced in U.S. Pat. No. 5,721,665 are not possible in the art of degaussing with permanent magnets.
The present invention treats all points within media to an extended range of magnetic directions, regardless of internal geometry, as a safeguard against fringing effects intrinsic to magnetic patterns encoding information on coatings of the magnetic media, by means not practiced in the art but particularly adaptable to the use of permanent magnets as field generators. In doing so, it can avoid the need for any electrical power, including even battery-powered process monitoring circuitry.
It is an object of the present invention to provide means and apparatus for bulk degaussing magnetic media which solves various problems in the prior art.
It is an object of this invention to provide means and apparatus to vary magnetic field direction while maintaining field strength in such a way as to expose all points within media to three-dimensional directions at effective strengths.
It is yet another object of this invention to produce such multi-directional strength with essentially zero energy input, excepting energy that may be provided by human operators to introduce media into the degausser, to extract media from it as needed, and for manual adjustments to accommodate different media thickness.
Briefly, the present invention relates to bulk degaussing apparatus, and more particularly to means which changes field direction using fixed arrays of magnet poles constructed with simple geometries and predisposed about a path provided for bulk degaussing media.